A major US public health issue is the use of psychostimulants, such as cocaine, which causes severe physical and psychological health problems. Genetic background differences in inbred mice affect their behavioral traits, and it is well known that inbred C57/BL/6J mice show less sensitivity to locomotor stimulant effects of cocaine and demonstrate high cocaine seeking behaviors. It was discovered that C57/BL/6J mice have a natural single base mutation in the gene encoding the serotonin metabolizing enzyme arylalkylamine N-acetyltransferase (5-HT N- acetyltransferase; AANAT), resulting in non-functioning protein. Because of this natural mutation in the AANAT gene, C57/BL/6J mice cannot synthesis NAS and melatonin from serotonin and can be considered a "natural AANAT knock-out" and/or "NAS/melatonin deficient." NAS and melatonin may act by binding to their receptors and/or modulating other neurotransmitter systems in the brain, such as dopamine, serotonin, and nitric oxide. We have demonstrated that AANAT-mutant C57BL/6J mice develop behavioral sensitization to repeated administrations of cocaine whereas AANAT-normal C3H/HeJ mice do not at night (high AANAT activity). We hypothesize that: a) brain and/or pineal AANAT gene expression is crucially involved in the neurobiological effects of cocaine (e.g. sensitization); and b) the AANAT/NAS/melatonin system might be a therapeutic target for agents that would prevent cocaine abuse. These hypotheses will be tested in the following three AIMs: (1) To investigate whether AANAT expression contributes to cocaine-induced behaviors; for this purpose, AANAT0mutant C57/BL/6J mice and an animal model of pinealectomy will be tested during the day and at night to measure cocaine-induced locomotor sensitization and cocain-associated drug seeking behavior; 2) to characterize the effects of repeated cocaine administration on the expression of neuronal AANAT and melatonin/NAS receptors; to understand the drug-caused long-term changes, we will measure the expression levels of AANAT and melatonin/NAS receptors; (3) to investigate whether pharmacological doses of NAS and/or melatonin and their receptor agonists and antagonists modulate cocaine-induced behaviors; since nocturnal (physiologic) increase in melatonin and NAS levels prevent development of sensitization, we will test whether pharmacological applications of selective agonists and antagonists, such as 5-MCA-NAT and 4p-PDOT, modulate cocaine-induced behavior. To test our hypothesis, molecular biological techniques (quantitative reverse transcription-polymerase chain reaction [RT-PCR] and in-situ RT-PCR), behavioral tests (locomotor activity and conditioned place preference), and pharmacological studies (agonists and antagonists for these receptors) will be used We expect to elucidate the role of the mammalian AANAT/NAS/melatonin system in the psychostimulant action of cocaine and to provide new therapeutic approaches for the prevention of the major components of drug addiction; i.e., sensitization and reward.